1. Field of the Invention
The present invention relates to a fixing apparatus for applying heat to paper carrying toner so that the toner is fused so as to be fixed on the paper. In particular, the present invention relates to a fixing apparatus that employs induction heating.
2. Description of the Prior Art
Heat rollers are widely used in electrophotographic image forming apparatuses. In a fixing apparatus employing a heat roller, a heat source is incorporated in at least one of a pair of rollers that forms a nip, and the pair of rollers is heated by that heat source. Paper carrying a toner image is passed through the nip between the pair of rollers so heated, so that the toner is fused so as to be fixed on the paper.
Fixing using a heat roller as described above is typically achieved with a construction in which a heat source such as a halogen lamp is built into a roller so that the heat generated by the heat source is conducted to the surface of the roller. This generally results in inefficient heat conduction to the roller surface and thus in a great loss of heat. Moreover, heating the roller surface to a sufficiently high temperature requires a long time. That is, quite inconveniently, low heat conduction efficiency results in high electric power consumption and in a long warm-up time, specifically requiring as long as several minutes for the roller surface to reach a sufficiently high temperature to achieve fixing.
For the purposes of increasing heating efficiency and reducing the warm-up time, there have been proposed fixing apparatuses that employ induction heating. For example, Japanese Patent Application Laid-Open No. 2000-268952 discloses a fixing apparatus in which, as exciting coils, a first and a second coil are arranged opposite to each other so that they are magnetically coupled together cumulatively. A carrier member having a heating layer inside it passes between the first and second coils. The heating layer is made of copper, silver, aluminum, or a material having an electrical resistivity equal to or less than those of the just mentioned metals, and is formed into a thin layer. The magnetic flux excited by the exciting coils penetrates the heating layer while describing loops, and causes magnetic induction, including eddy currents in the heating layer. These eddy currents produce Joule's heat, with which the heating layer is heated.
The fixing apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-268952 mentioned above requires two exciting coils, i.e., the first and second coils. This makes this fixing apparatus comparatively expensive and large.
Moreover, in the fixing apparatus described above, while the carrier member is provided with a heater, the pressure member that forms a nip between itself and the carrier member is not provided with a heater. Thus, the pressure member is heated only with the heat it receives from the carrier member. Even once the pressure member is heated to a temperature close to that of the carrier member, as paper is passed, it snatches away the heat of the pressure member, making it less hot immediately. To recover the temperature of the pressure member, it needs to be brought into contact with the carrier member again. However, as long as paper is fed continuously, it is impossible to secure a sufficient time for their contact. Ultimately, the pressure member may remain less hot, resulting in poorer fixing performance than is expected.
A heating member for induction heating is commonly formed of a magnetic metal. However, it is known that even a non-magnetic metal offers higher heating efficiency than a magnetic metal, provided that the heating member is made sufficiently thin. The fixing apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-268952 includes a construction in which a thin layer of a non-magnetic metal is used as a heating layer. However, this construction cannot be said to achieve heating by fully exploiting the properties of a non-magnetic metal, which is inherently difficult to heat by induction heating.
In the fixing apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-268952 mentioned above, the exciting coils are arranged so as to sandwich the carrier member from both sides. To realize this construction, the exciting coils need to be located away from the nip, through which paper is passed. This causes heat to escape from the heated part of the carrier member before it reaches the nip, and thus the energy fed from the exciting coils to the heating member is not efficiently conducted to paper. Making an allowance for the expected drop in temperature when setting the target temperature to which to heat the carrier member leads to increased electric power consumption. How heat is dissipated from the carrier member depends on the ambient conditions such as temperature and humidity, and therefore, if there is a long distance from the exciting coils to the nip, it makes unstable the temperature of the carrier member as it passes through the nip.
Moreover, when a heating layer made of a non-magnetic metal is heated by induction heating, the magnetic field produced by exciting coils is transmitted through the heating layer, with the result that metal components located nearby are heated unnecessarily, leading to a waste of energy and an unduly high temperature inside the apparatus. In the fixing apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-268952, an attempt is made to prevent leakage of the magnetic flux by arranging the exciting coils so as to sandwich the heating layer. The effect of this arrangement, however, cannot be said to be satisfactory.
In a case where electromagnetic induction heating is applied in a fixing apparatus provided with a fixing roller and a pressure roller, and where exciting coils are built into the fixing roller, the interior of the fixing roller becomes hot owing to the heat radiated from the heated fixing roller itself and the heat dissipated from the exciting coils. The exciting coils are typically combined with a ferrite core for the purpose of intensifying the magnetic field. Ferrite changes its properties according to temperature, and loses one of its characteristic properties, namely high magnetic permeability, at temperatures over 200° C. This may make it impossible to achieve the desired intensification of the magnetic field.